Before the availability of lasers capable of providing large numbers of photons at specific desirable frequencies and at high densities, gaseous diffusion or ultra-centrifugation (UCF) were the preferred processes for high volume isotope separation processes, with the enriching of uranium to reactor-grade concentrations being the highest volume process. These mass-action processes, which depend on the small mass differences between naturally occurring isotope-carrying molecules, are well developed and today are used to provide the enriched uranium for most operating nuclear power plants.
Quantum processes that use lasers are inherently more efficient and are expected to replace diffusion and UCF processes some time in the future. Three different approaches for the laser isotope separation of uranium, which show promise as the next preferred commercial separation process have emerged after two decades of intensive research beginning in 1970. These processes are known by the acronyms AVLIS for Atomic Vapor Laser Isotope Separation, MOLIS for Molecular Obliteration Laser Isotope Separation, and CRISLA for Chemical Reaction by Isotope Selective Laser Activation.
In the AVLIS process, uranium is vaporized by bombarding molten uranium with an electronic beam. .sup.235 U atoms in the vapors are preferentially excited and ionized in three steps by laser photons in the visible spectrum: ##STR1## Superscripts .sup.e * and .sup.v * indicate electronic and vibrational excitations respectively. One * designates a general or a single excitation, ** a double excitation, etc. The ionized uranium atoms, .sup.235 U.sup.+, are passed through an electrostatic field which deflects them to collectors while the un-ionized depleted uranium flows on and condenses out for subsequent removal from the process. The AVLIS process requires expensive isotope handling and vaporizing equipment and therefor is not practical when small quantities of isotopes need to be separated, such as the separation of molybdenum.
In the MOLIS process, a nozzle-cooled flowing gas of UF.sub.6 diluted with N.sub.2, Ar, He, H.sub.2, and/or CH.sub.4, is dissociated to UF.sub.5 +F by exposure to photons at three different successive frequencies. The photons usually are generated by two fine-tuned far-infrared (16 .mu.m) lasers and one ultraviolet laser. Instead of a UV laser, a high-intensity far-infrared laser can also be used for the final step. The lasers are tuned to preferentially excite and dissociate .sup.235 UF.sub.6 but since the absorption bands of .sup.235 UF.sub.6 and .sup.238 UF.sub.6 at the required temperature and pressures somewhat overlap, some .sup.238 UF.sub.6 is also undesirably disassociated. The consecutive process steps required to disassociate .sup.235 UF.sub.6 can be written symbolically: ##STR2## Following dissociation by step (6), solid UF.sub.5 out of a gaseous mixture of .sup.238 UF.sub.6 and .sup.235 UF.sub.5 is collected on an impact plate. Contact with the impact plate is assured by first passing the gaseous UF.sub.6 /UF.sub.5 mixture through a plate perforated with holes that is positioned just upstream from the impact plate to force the gas to make a 90.degree. turn. Solidified UF.sub.5 piles form opposite the holes while gaseous UF.sub.6 passes on.
Often a so-called "scavenger" gas like CH.sub.4 is added to the flow to remove F radicals and to prevent the back-reaction UF.sub.5 +F.fwdarw.UF.sub.6. With the current state of the laser art, the photon frequencies and intensities required for the MOLIS process can only be produced by pulsed lasers. Since the UF.sub.6 to be excited moves across the laser beams, the pulse repetition rates of the pulsed lasers must be high enough so that most of the gas flowing by is laser-irradiated. Otherwise an insufficient fraction of the UF.sub.6 will experience excitation, causing the MOLIS process to be inefficient. Also, because multiple lasers at different specific frequencies are required, MOLIS is not economic to use to separate small quantities of isotopes where the cost of equipment must be amortized, although for such purpose, MOLIS is much more economic than AVLIS.
In a uranium CRISLA process, gaseous .sup.235 UF.sub.6, diluted with a carrier gas such as N.sub.2, Ar, He, or H.sub.2, is preferentially excited by irradiation with infrared photons. The reaction cell may be placed inside the cavity of a CO laser, which can then excite the 3.nu..sub.3 vibration in UF.sub.6 with its 5.3 .mu.m photons in one step. Such a CO laser can be operated continuously. The UF.sub.6 may also be step-wise excited to a multi-quantum level by 16 .mu.m laser photons from a pulsed or CW laser. A gaseous coreactant RX is mixed with the UF.sub.6 either before or after its laser irradiation. With suitable CRISLA coreactants RX, the reaction rate of laser-irradiated .sup.235 UF.sub.6 is greatly enhanced over the thermal chemical reaction rate of .sup.238 UF.sub.6. This rate enhancement is given by the factor ##EQU1## where .rho..sub.a is the statistical weight of molecular vibrations that promote the reaction, h.nu..sub.L is the laser photon energy, and kT is the thermal Boltzmann energy of the gas. For CO lasers with h.nu..sub.L =1876.3 cm.sup.-1, ##EQU2## With typical values of .rho..sub.a =56, .theta..sub.L =140 at T=300.degree. K, .theta..sub.L =1.2.times.10.sup.6 at T=200.degree. K, and .theta..sub.L =1.times.10.sup.10 at T=100.degree. K. Thus lower operating temperatures give higher laser-enhancement rates in CRISLA.
The basic process steps in uranium CRISLA are: ##STR3## Even though the laser frequency .nu..sub.L is tuned to the peak absorption for .sup.235 UF.sub.6, because of the partial overlap of the absorption bands of .sup.235 UF.sub.6 and .sup.238 UF.sub.6, some .sup.238 UF.sub.6 is also undesirably excited. The enriched reaction product (UF.sub.4, UF.sub.5, or UF.sub.m X) in (8) either has a lower vapor pressure than UF.sub.6 so that it can be removed from the gas mixture by differential freezing, or it polymerizes or decomposes into a solid precipitate that can be removed by mechanical and/or chemical means.
In a molybdenum CRISLA process to separate radioactive .sup.99 Mo from a mixture of .sup.99 Mo and .sup.98 Mo, the molybdenum mixture is fluorinated and the resultant gaseous .sup.99 MoF.sub.6, diluted with a carrier gas, is preferentially excited over the gaseous .sup.98 MoF.sub.6 by irradiation with infrared photons that can be produced by a 9 .mu.m CO.sub.2 laser that selectively excites the .nu..sub.3 +.nu..sub.5 vibration in .sup.99 MoF.sub.6.
Typical CRISLA processes are described in U.S. Pat. Nos. 5,110,430; 5,108,566; 5,015,348; and 4,082,633, all by Jozef W. Eerkens, and U.S. Pat. No. 4,948,478 by Alexander Obermayer, which are incorporated herein by reference. The use of lasers to selectively excite a desired isotope is possible whenever the spectral absorption peaks of the isotope and/or compounds thereof to be separated occur at small frequency differences from the other isotopes in the mixture. These isotope frequency shifts are caused by the different masses of the isotopes that affect internal electronic and vibrational frequencies. For the purely electronic absorption lines of atomic U, there is not only an isotope shift of approximately 0.2 cm.sup.-1, between .sup.235 U and .sup.238 U, but also a nuclear-spin-induced splitting of the .sup.235 U absorption into eight hyperfine lines spread out over approximately 0.15 cm.sup.-1 while .sup.238 U, with an even number of nucleons, has only one absorption line. In the case of UF.sub.6 molecules, the isotope shift between .sup.235 UF.sub.6 and .sup.238 UF.sub.6 is about 0.6 cm.sup.-1 for the strongest stretching vibration .nu..sub.3 (near 16 .mu.m) and 1.8 cm.sup.-1 for the tertiary 3.nu..sub.3 absorption (near 5.3 .mu.m). Similarly, for .sup.98 MoF.sub.6 and .sup.99 MoF.sub.6, the isotope shift of both the .nu..sub.3 and the .nu..sub.3 +.nu..sub.5 band is approximately 1.0 cm.sup.-1. At room temperature the .nu..sub.3 and 3.nu..sub.3 absorption bands of .sup.235 UF.sub.6 are spread over approximately 15 cm.sup.-1 and to some extent overlap the absorption bands of .sup.238 UF.sub.6. However at lower temperatures, the band spreads become narrower and the two isotopic bands become essentially separated below about 100.degree. K. This spreading and overlapping of the bands occurs in most isotopic mixtures of medium to heavy molecules such as QF.sub.6, if the atomic mass of Q exceeds about 50 amu.
For MOLIS and CRISLA processes that use gaseous QF.sub.6, higher separation factors can be achieved if the QF.sub.6 is cooled from 300.degree. K to temperatures between 10.degree. K and 100.degree. K Also for CRISLA, laser-induced reaction rates are considerably enhanced at lower temperatures over thermal rates. The QF.sub.6 cooling can be accomplished by expansion through a supersonic nozzle before laser irradiation. Although QF.sub.6 is normally a solid at very low temperatures, when QF.sub.6 is diluted in a carrier gas and subjected to supersonic expansion, the QF.sub.6 remains gaseous at 10.degree. K&lt;T&lt;100.degree. K for the .about.0.1 milliseconds it takes to traverse the downstream section of a supersonic nozzle.
In the conventional uranium MOLIS process, the supercooled UF.sub.6 flow is cross-irradiated by two pulsed 16 .mu.m laser beams of moderate power and by a pulsed dissociation-producing UV or high-intensity IR laser beam. The three different laser pulses usually have 10 to 100 ns durations and follow each other within micro-second time intervals or partially overlap. The pulse repetition rate (prr) of the three companion pulses must be high enough so that the cross-flowing UF.sub.6 is struck at least once as it flows by. If the transit time is .tau..sub.R the pulse rate must be at least 1/.tau..sub.R. Otherwise only a small fraction of the .sup.235 UF.sub.6 that flows through the nozzle is excited and the UF.sub.6 must be recycled through the nozzle many times. Dicke superradiance and other losses during .sup.235 UF.sub.6 laser-pumping do not allow all the .sup.235 UF.sub.6 to be excited in one pulse and make it necessary to further increase the minimum pulse rate of 1/.tau..sub.R .apprxeq.10,000 Hz. The high prr requirements for 16 .mu.m MOLIS lasers have pushed the limits of existing pulsed laser technology. With the present state-of-art, some ten or more 16 .mu.m lasers would have to be multiplexed to get the desired result, unless enrichment is carried out in ten or more stages.
Ultimately, the most advantageous isotope separation system is that system that can produce a given amount of separation for the lowest overall cost. All the above processes work, but the CRISLA process appears most economic both for high volume separation of uranium and production of microgram quantities of radioisotopes for medical uses. The main reason is that the MOLIS process, which is the closest contender to the CRISLA process, requires expensive laser energy to supply all the separation energy, whereas in the CRISLA process, expensive laser energy is used only for the activation of an isotope-specific reaction. Most of the isotope separating energy in CRISLA is provided by inexpensive chemical energy so that when sufficient enrichment is not achieved in one pass, economics has allowed multiple serial CRISLA processes to be proposed. Also for CRISLA, only one laser (usually CO or CO.sub.2) with one output frequency and a single-step isotope-selective excitation is usually sufficient. Additional multi-step booster excitations may be advantageous under some circumstances to provide adequate energies for overcoming subsequent chemical reaction barriers. In the MOLIS process, two or three different laser isotope-selective frequencies and at least two different pulsed lasers are required. In a single-step CRISLA process, a CO or CO.sub.2 laser can be operated continuously, whereas multi-step MOLIS lasers need accurately timed pulses. The 16 .mu.m MOLIS lasers must use Raman conversion cells filled with para-H.sub.2 thereby adding one additional piece of optics-loaded hardware and an additional special gas. Also in the MOLIS process, all optical windows must be made from expensive ZnSe and RbCl to allow transmissions at 16 .mu.m, whereas in a CRISLA process that uses 5.3 .mu.m or 9 .mu.m radiation, less expensive CaF.sub.2 and KCl windows can be used. Dicke superradiance losses and high prr problems are also absent in a CRISLA process that uses single-step 5.3 .mu.m or 9 .mu.m excitations from a continuous CO or CO.sub.2 laser.
In some cases it may be advantageous in CRISLA to employ isotope-selective pulsed 16 .mu.m multi-step-excitation lasers similar to those used in the MOLIS process, in spite of the prr problems just mentioned. Even then, the CRISLA process is less expensive than MOLIS since laser excitation of the isotope is not carried out all the way to the dissociation limit and chemical energy is substituted for laser photo-dissociation energy. Of course CRISLA uses coreactant chemicals that are absent in the MOLIS process (MOLIS does use chemicals in the product removal phase). However, chemicals are relatively inexpensive and their consumption costs are less than those of laser photo-dissociation. Laser hardware and maintenance costs have a much larger effect on the unit enrichment cost than the cost of chemicals, and some of those can be recycled in the CRISLA process. This is clearly indicated in detailed cost analyses of uranium enrichment by the MOLIS and CRISLA processes that show CRISLA to be the most economic.
Although each of the three uranium laser enrichment processes (AVLIS, MOLIS, and CRISLA) appears conceptually straight-forward and reasonably simple to carry out, in practice all three have proven to be more complicated and costly to implement than was originally anticipated.
For AVLIS, after finding suitable high-power visible lasers and tunable frequencies, the main technical problem is the development of manageable processes and durable materials to handle molten Uranium and Uranium vapor. For MOLIS, the major technical problem is the development of new, efficient, reliable, pulsed lasers at the desired wavelengths (pulsed because CW lasers are not available at the desired wavelengths) and at sufficiently high pulse repetition rates (prr). To attain one-stage enrichment with 16 .mu.m pulsed lasers for example one needs a prr of more than 60,000 Hz, whereas the most advanced units today can only provide 4000 Hz. This results in the requirement to multiplex ten or more 4000 Hz sets of pulsed 16 .mu.m lasers (each set with two or three laser chains) to provide the necessary irradiation time coverage and .sup.235 U depletion for a one-stage process. For CRISLA the main problem is to find one or more suitable coreactants and to understand the chemistry.
In both MOLIS and CRISLA, another development problem has been the efficient separation of condensing enriched product .sup.e UF.sub.5 (X) from the depleted .sup.d UF.sub.6 gas stream and the product removal from collector surfaces. It was found that laser-produced enriched products such as .sup.e UF.sub.5 (X) can back-react on UF.sub.6 -covered surfaces by the reaction: EQU .sup.e UF.sub.5 (X).dwnarw.+UF.sub.6 :Wall.fwdarw..sup.e UF.sub.6 (g).uparw.+UF.sub.5 :Wall+(1/2X.sub.2 :Wall) (9)
When a mixed gas stream containing UF.sub.6, UF.sub.5 (X), RX, and carrier gas (e.g. N.sub.2) flows past an untreated collector surface, UF.sub.6 tends to be adsorbed on the surface in addition to the condensing {UF.sub.5 (X)}.sub.n (n=1, 2, 3, . . . for the condensing or polymerizing UF.sub.5 (X)). Since UF.sub.6 is usually present in excess over UF.sub.5 (X), reaction (9) can take hold quickly. Besides reaction (9), even if a passivated wall has no UF.sub.6 adsorbed on it, the reverse of (9) can take place after several monolayers of solid {.sup.e UF.sub.5 (X)}.sub.n have precipitated out on it: EQU .sup.e UF.sub.5 (X):Wall+UF.sub.6 (g).fwdarw..sup.e UF.sub.6 (g).uparw.+UF.sub.5 :Wall+(1/2X.sub.2 :Wall) (10)
Both U-exchange reactions (9) and (10) undo the original laser-induced isotopic change that was achieved and are referred to as isotope "scrambling" reactions. To overcome the isotope scrambling reactions (9) and (10), there are several remedies. These special product harvesting techniques form part of the present invention and are described in detail in what follows.